(CNN) -- The number of North Koreans who have defected to South Korea has passed 20,000, the Yonhap news agency said Monday.

A 41-year-old woman identified only by the last name Kim arrived in South Korea on Thursday, becoming the 20,000th defector, Yonhap said, citing South Korea's Unification Ministry.

The woman fled with her sons, ages 12 and 17, because of economic hardships in the North, Yonhap said.

The family arrived in South Korea via China, but no further details were available, the agency said.

An average of 200 North Koreans arrive in the South monthly, according to the Unification Ministry.

North Koreans also escape to neighboring China, but their numbers are unknown, because they go underground. Beijing sends North Korean defectors back to the hermetic North, where they face punishment.

Many North Koreans also are caught while trying to escape their impoverished country.

Millions of families were separated by the Korean War, which ended in 1953 with a ceasefire, but without a formal peace treaty.

South Korea turned itself into an economic tiger in the decades after the war. It overtook the communist North, which was industrially superior at one point.

North Koreans have gone hungry as their leaders have poured money into the country's nuclear arms and missile programs, analysts say. The arms buildup has prompted harsh economic sanctions from the international community.

Weather-driven food shortages have compounded the hardships faced by North Koreans.